We are investigating the use of the EPR (electron paramagnetic resonance) absorptions of rare earth and other paramagnetic metal ions bound to organic molecules in crystals for ENDOR (electron nuclear double resonance) spectroscopy of nuclei, in particular, protons, located near the binding site. Our long term objective is to obtain from such spectra precise values of the coordinates of hydrogen atoms in large molecules such as proteins (enzymes) by means of measurements of the variations of ENDOR frequencies with direction of the magnetic field relative to the molecule. The most common method of determining the structures of such molecules, namely, x-ray diffraction, does not usually yield this information. Successful application of this technique would permit the quantitative study of the changes in local structure of proteins when metal ions are bound, the determination of hydrogen atom positions at the active sites of enzymes, the study of details of the conformational changes which occur during the binding of substrates, the investigation of the role of hydrogen bonds in the attachment of the substrates, and the study of the part played by protons in enzyme catalyzed reactions. We have demonstrated the feasibility of this technique with model systems and are currently applying it to enzymes and other proteins.